1. Field of the Invention
The invention relates to a method of managing the valves of a simulated moving bed separation system, allowing finer monitoring of a determined concentration profile.
2. Description of the Prior Art
As already described in French patent 2,762,793 filed by the assignee, a simulated moving bed (SMB in abbreviated form) separation system, also referred to as simulated countercurrent (SCC in abbreviated form) separation system, comprises (FIG. 1) a series of beds filled with adsorbent which are arranged in a closed loop. A fluid circulation is established through this loop. The loop or column is divided into a succession of zones (Z1 . . . Z4), generally four, each zone consisting of a certain number of beds. Lines are connected to the loop between the various zones, allowing injection of a feedstock (A+B) at least one of the components of which is to be separated and of an eluent (S) which mainly contains the desorbent, or draw-off of an extract (Ex) which mainly contains the preferably adsorbed component or of a raffinate (Raf) which mainly consists of the least preferably adsorbed elements.
The term adsorbent is used here in its most general sense. It can be an adsorbent such as a molecular sieve, zeolitic for example, or an adsorbent of ion-exchange resin type.
The extract can contain, in addition to the preferably adsorbed product, other products circulating in the column that will have to be separated in distillation columns outside the adsorption column, thus allowing obtaining the product or products at the required purity level.
In the case of a real countercurrent (RCC) separation system, a fixed and constant concentration profile develops along separation column 1. The position of the injection points of feedstock A+B, of eluent S, and of the draw-off points of extract Ex and of a raffinate Raf remains fixed. Adsorbent solid 3 and fluids 2 circulate in a countercurrent flow. A solid driving system and a recycle pump P, both arranged on the location of the column (at the junction of zones Z1 and Z4 where the only species present, in the liquid as well as in the solid, is the eluent), respectively allow sending the solid back from the base to the top, and conversely the liquid from the top to the base.
In the case of a simulated countercurrent (SCC) separation system, the injection points of the feedstock and of the eluent and the draw-off points of the raffinate and of the extract are periodically moved forward in the direction of circulation of the fluids. All of the injection and draw-off points move forward at each period by the same increment of a length equal to one bed, so that the length of each zone or, which is equivalent, the number of beds per zone, remains unchanged.
A concentration profile of the various species present, which is going to move along the loop mainly according to the recirculation flow which is entirely recycled to the top or to the bottom of the column depending on the direction of circulation selected, generally from the top of the column downwards, is established in the column.
All the injection and draw-off lines of a connection separating two consecutive zones are thus shifted simultaneously at each period ΔT upwards or downwards according to the direction of circulation of the recirculation flow, and after a certain time called cycle time, they are back in their initial position.
If all the connections are numbered from 1 to N, for example from the top to the bottom of the column, the position of the feedstock and eluent injection points and of the raffinate and extract draw-off points can be defined at each period by assigning to each flow the number of the connection corresponding thereto.
For example, S (1)/Ext (5)/Feedstock (9)/Raff (13) means that the feedstock is injected at connection 9, that eluent S is injected at connection 1, that raffinate Raf is drawn off at connection 13 and that extract Ex is drawn off at connection 5.
The column diagrammatically shown in FIG. 1 comprises 15 connections, delimiting 15 beds, and each connection is provided with at least one injection line and at least one draw-off line.
At the time T0, the positions of the eluent, of the extract, of the feedstock and of the raffinate are thus: 1, 5, 9, 13. At the time T0+ΔT, the positions all increase by one increment and respectively become 2, 6, 10, 14, at the time T0+2*ΔT, they become 3, 7, 11, 15, etc. It is understood that, after the time T=T0+15ΔT, the positions of the injection points and of the draw-off points are back in their initial position.
The number of beds assigned to each zone can also be determined. Zone 1 being defined as located between the eluent injection line and the extract draw-off line therefore comprises 5−1=4 beds. Similarly, zone 2 is defined as located between the extract draw-off line and the feedstock injection line, and comprises 9−5=4 beds. Zone 3 is defined as located between the feedstock injection line and the raffinate draw-off line and comprises 13−9=4 beds. Zone 4 is defined as located between the raffinate draw-off line and the eluent injection line and comprises 3 beds.
In brief, the characteristics of a simulated moving bed are defined by the zones which comprise each a determined and whole number of beds, and a synchronized displacement by one bed at each period ΔT of all the injection and draw-off lines, which allows to maintain the length of each zone constant.
Selection of the period ΔT and of the injection and draw-off flow rates is determined so as to maximize the purity of the product to be upgraded at a given yield, or conversely to maximize the yield for a fixed purity as described in the aforementioned French patent 2,762,793.
A SCC separation system has two drawbacks inherent in the concept of simulated moving bed which causes continuous displacement of the concentration profiles of the various species present in the column in relation to the RCC system wherein these profiles would be stationary:
1) the length of a zone can only be a whole number of beds because a zone is precisely defined by the connections that surround it, therefore by the number of beds contained between said connections, and
2) during the draw-off period, the profile moves before the draw-off point by the length of a bed, which leads to draw off over a certain length of a bed around the optimum point, without being permanently on this optimum point precisely, as it would be the case with a RCC since the profiles are stationary in this case.
French patent 2,785,196 1 (EP-1,128,881) filed by the assignee describes a method allowing improvement of the purity of the raffinate or of the extract by dividing the position shift period into several subperiods during which the length of certain zones can vary.
In each subperiod, a zone has a decreased length and another zone has a length increased by the same quantity, so that the total length of the zones or length of the column remains the same.
At the next subperiod, the zone whose length has been decreased remains the same, but the zone whose length has been increased moves forward to become the immediately consecutive zone in the direction of flow of the fluids in the loop, and so forth up to the initial situation at the end of the period.
Everything goes off as if, over the period, the column had worked on average with zones of lengths different from their physical lengths. During the period, it is therefore all of the connections that are concerned by the displacement of the injection and draw-off points, which in practice leads to an extremely delicate and high-frequency management of the valves controlling injection and draw-off.
Furthermore, the method of operation described in the aforementioned French patent 2,785,196 is based on a completely separate opening and closing of the consecutive valves insofar as there is no temporal overlap. Each injection (respectively each draw-off) is carried out through a single injection (respectively draw-off) point.
French patent 2,721,528 and corresponding U.S. Pat. No. 5,578,215 describes a method allowing improvement of the purity of the raffinate or of the extract by slightly changing the shift period of the injection or draw-off positions of each bed in relation to a fixed value ΔT so as to take account of the molecular sieve filling differences of each bed or of the adsorbent phase quantity differences from one bed to the next.
The method described in this patent permutates the injection and draw-off streams independently of one another and at programmed time intervals so that, after N permutations, each stream has passed through the whole of the closed loop.